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Issue 133 | Mar – Apr 2018 | GRDC GROUNDCOVER SUPPLEMENT: CROP SEQUENCING
GROUNDCOVER
PULSES
Simple ‘rules of thumb’
allow growers to predict
the nitrogen benefit for
their specific situation
By Dr Mark Peoples
n Growers with cereal-based crop sequences
sometimes view legume break crops as
too risky for the income they generate.
However, a five-year research project across
south-eastern Australia found that the
benefits of including a legume break crop
are more significant than anticipated.
It found that legume break crops are
as profitable, and in many cases more
profitable, than wheat based on the grain
prices and growing seasons experienced
during the project, from 2010 to 2015.
The GRDC research investment was
led by CSIRO in conjunction with the
NSW Department of Primary Industries,
Agriculture Victoria, the South Australian
Research and Development Institute
and seven grower groups across the low,
medium and high-rainfall zones and
irrigation areas. By combining the data
from numerous researcher and on-far m
trials they found that cropping sequences
that included at least one legume
break crop were more productive and
profitable than continuous wheat when
best management practices were used.
NITROGEN BENEFIT
Without an easy way to predict the
nitrogen benefit from including a legume
break crop growers tend to focus on the
simpler question ‘How much money can
I earn in the year of the break crop?’
Yet the results from these trials,
in combination with additional
findings from more than 25 years of
legume nitrogen research, revealed
that growers often underestimate the
nitrogen boost that legumes provide.
Soil mineral nitrogen (nitrate and
ammonium) measured in autumn to
a depth of 1.2 metres (the expected
rooting zone of a wheat crop) was,
on average, 35 kilograms of nitrogen
per hectare higher following a legume
SEQUENCING TRIALS TALLY
LEGUMES’ NITROGEN BENEFIT
crop than following a wheat or canola
crop. Results were even higher when
the legume was brown manured.
To help growers estimate the value of soil
nitrogen benefits researchers have developed
some potential predictive relationships
that growers and their advisers can use
as rules of thumb to assist in nitrogen
decision-making when sowing a cereal
crop after legumes (see ‘Rules of thumb’).
As most growers routinely measure
rainfall and grain yield the relationships
0.15kg of nitrogen per millimetre of
fallow rainfall, and 18 × legume grain
yield (tonnes per hectare) are perhaps
the simplest for growers to use.
The most reliable predictions were based
on legume residue nitrogen. The analyses
indicated that it explained the largest
fraction (57 per cent) of the observed
variation. However, residue nitrogen is a
particularly difficult parameter for growers
to measure directly. But since grain yield
is usually closely related to above-ground
residue biomass (shoot residue dry matter
as about twice the legume grain yield),
it was argued that grain yield might
also provide a guide to the amount of
total residue nitrogen remaining at the
end of the legume growing season.
Further analyses of experimental
data confirmed this relationship and
estimated the additional mineral
nitrogen as 0.28 × [54 + (30 × tonnes of
legume grain harvested per hectare)].
RECOVERY
Subsequent wheat crops recovered
an average of 30 per cent residual
legume nitrogen, which is comparable
to the amount recovered from fertiliser
nitrogen applied at sowing, but lower
than top-dressed fertiliser nitrogen (65
per cent of the 51 to 75kg fertiliser-N/
ha) applied at stem elongation just prior
to the peak period of nitrogen demand.
However, unlike fertiliser nitrogen,
legumes contribute to a large pool of
organic nitrogen that continues to benefit
more than one subsequent crop and
sustains soil fertility in the long term. o
GRDC Research Code CSP00146
More information: Dr Mark Peoples,
CSIRO Agriculture & Food, 02 6246 5447,
mark.peoples@csiro.au
CSIRO’s Tony Swan compares the growth of wheat following either a previous wheat (left) or
field pea crop (right) in the project’s Birchip Cropping Group trial at Hopetoun, Victoria.
PHOTO:MARKPEOPLES
RULES OF THUMB
Rules of thumb for estimating the
additional mineral nitrogen after
legume break crops:
n 0.15kg N/ha per millimetre of summer
fallow rainfall;
n 18kg N/ha per tonne of legume grain
harvested per hectare; or
n 0.28 × [54 + (30 × legume grain
harvested (t/ha))].